1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing information with respect to information recording medium such as an optical disk, etc.
2. Description of the Related Art
A general apparatus for recording and reproducing optical information has a laser beam source such as a semiconductor laser, a collimator lens for forming a light beam emitted from the laser beam source as an approximately parallel light beam, and an objective lens for converging the parallel light beam from the collimator lens in the shape of a spot on a recording face of an optical information recording medium such as an optical disk. In this apparatus, the information are recorded and reproduced by converging the laser beam in the shape of a spot on the recording face of the optical information recording medium.
In such an apparatus, to record and reproduce the information of high density, it is sufficient to reduce the distance between tracks of the optical information recording medium and the distance between pits. In this case, it is necessary to reduce the diameter of the light spot converged on the recording face of the optical information recording medium by the objective lens.
The diameter .phi. of the light spot is determined by a wavelength .lambda. of the laser beam and a numerical aperture NA of the objective lens and is represented as follows. EQU .phi..alpha..lambda./NA
Namely, the diameter of the light spot is decreased as the wavelength .lambda. of the laser beam is decreased and the numerical aperture NA is increased.
In general, a semiconductor laser (LD) is used as the laser beam source of the apparatus for recording and reproducing optical information. Accordingly, the wavelength .lambda. of the laser beam is normally ranged from 780 to 830 nm so that it is impossible to greatly narrow this range.
Further, as the numerical aperture NA of the objective lens is increased, the depth of focus of the objective lens is decreased so that the influence of inclination of the recording face of the optical information recording medium, etc. is increased. Therefore, it is necessary to perform an aligning operation of a focusing position with high accuracy so that there is a fear that the converging degree of the light spot is reduced. Further, when the numerical aperture NA is increased, the number of constructional elements of the lens is increased and cost is increased.
To solve the above problems, for example, in a technique proposed in Japanese Patent Application Laying Open (KOKAI) No. 63-247920, it is possible to form a small light spot by partially interrupting the light beam near an optical axis of the lens even when the light source having the same wavelength .lambda. and the objective lens having the numerical aperture NA are used.
However, in such an apparatus, the small light spot cannot be formed without loss of quantity of light emitted from the laser beam source.